1. Field of the Invention
The present invention relates to a superconductor for magnetic field shield which shields magnetic fields using superconductive materials.
2. Prior Art
As a magnetic field shield utilizing superconductivity, the first class superconductor or the second class superconductor has been used depending on the intensity of a magnetic field. The magnetic field shield using the first class superconductor utilizes perfect diamagnetism (Meissner effect), a characteristic of superconductivity. This magnetic field shield cannot shield intense magnetic fields since its critical magnetic flux density is low. The magnetic field shield using the second class superconductor utilizes the above-mentioned perfect diagmagnetism and the diamagnetism obtained by a mixture of the superconductor state and the normal conduction state. The critical magnetic field is separated into the upper and lower critical magnetic fields. Since the intensity of the upper critical magnetic field is extremely high, the magnetic field shield using the second class superconductor can be used to shield intense magnetic fields.
Superconductivity shielding and electromagnetic shielding are used to shield magnetic fields using superconductors. Superconductive shielding uses the perfect diamagnetism (a characteristic of superconductivity) and the diamagnetism obtained by the above-mentioned mixture condition. In the case of electromagnetic shielding, what is called interlinkage magnetic flux unchangeability principle is used to generate magnetic fluxes, the direction of which is opposite to that of the magnetic fluxes interlinking in a closed circuit obtained by connecting conductors one after another.
As a magnetic field shielding application example of the above-mentioned second class superconductor, a superconductive sheet or tape wound around a cylindrical core material is used. An example of this type is disclosed as the Japanese Provisional Patent Publication No. 56-40289. This magnetic field shield is disposed in an intense magnetic field to shield the internal space of the core material against external magnetic fields, or used to prevent the magnetic field of a magnet disposed in the core material from leaking outside.
The U.S. Pat. No. 3,281,738 discloses a superconductive solenoid. In this superconductive solenoid, discs on which superconductive rings are formed coaxially and discs made of a material superior in thermal and electrical conductivity are laminated alternately to form a cylinder. This cylinder is intended to be used as a magnet by taking magnetic fluxes inside. It can also be used as a magnetic field shield since it contains superconductors between the internal and external spaces.
The shield composed of a cylindrical core material on which the above-mentioned superconductive sheet or tape is wound is used to electromagnetically shield the internal and external sides of the core material via the junction at the ends of the superconductive sheet or the junctions at the fringes of the superconductive tape. Therefore, the junction condition greatly affects the magnetic field shielding effect. The above-mentioned Japanese Patent Provisional Publication discloses a mehtod wherein a superconductive sheet is wound around a core material and dipped in a melted metal (with a low melting point) to join the fringes of the superconductive tape. In this case, however, the melted metal is not fully distributed and the thickness of the metal layer is not uniform. As a result, the shield has a low shielding effect to a magnetic field parallel to the axis of the core material and the effect reduces secularly. When the shielding effect on the surface area of the shield to a magnetic field is examined, no electrical interlinkage condition by the superconductive tape is not formed since the low melting point metal discontinues at some portions. The interlinkage magnetic flux unchangeability principle cannot function sufficiently. In addition, differences in electrical resistance are caused due to differences in thickness of the low melting point metal. At a thick portion, Joule heat generates and the electrical interlinkage condition is apt to be lost secularly.
As another example, a net tape including a superconductive wire material is wound on a cylindrical core material and joined using Wood's metal or solder. This shield has numerous junctions and its magnetic field shielding effect reduces secularly due to the electrical resistance generated at the junctions.
When the superconductive solenoid of the above-mentioned U.S. Patent is used as a magnetic field shield, it is estimated to be superior to the above-mentioned shield in the shielding stability and secular shielding characteristics. The superconductive disc of the solenoid is made by coating numerous coaxial rings (ring width: 0.02 to 0.16 cm) of a superconductive material (NbTi for example) on at least one side of a metal substrate. The ring width is set to 0.16 cm or less. If it exceeds 0.16 cm, eddy current generates and the intensity of the magnetic field to be trapped is reduced. The multiple coaxial rings formed on the disc ensure the total magnetic field trap amount for a single superconductive disc. When the disc is examined in the viewpoint of magnetic field shielding, the narrow width of the superconductive material reduces the magnetic field shielding effect. Therefore, a large structure is required to obtain even a small shielding space. Accordingly, the above-mentioned superconductive solenoid is hardly applicable to a magnetic field shield. The superconductive discs and the above-mentioned metal discs are alternately laminated. Because grooves are present between the superconductive rings, magnetic fluxes enter via the metal discs and grooves when the thickness of the metal discs is increased. To prevent this problem, the thickness of metal discs should be as small as possible. However, when thin discs are used in a magnetic field shield, the proper shielding space of a shield structure cannot be adjusted easily depending on the size of the object to be shielded.